This invention relates to the biological treatment of wastewater, and more particularly to an improved form of rotating biological contactor. Rotating biological contactors (RBC) generally comprise a cylindrical framework with a labyrinthine interior media designed to provide extensive air/water contact surfaces. The cylinder rotates about a horizontal axis in a secondary wastewater treatment tank. RBCs provide surfaces for the growth of a biomass which has the ability to adsorb, absorb, coagulate and oxidize undesirable organic constituents of the wastewater and to change them into unobjectionable forms of matter. The contactors are typically rotated partially submerged in a wastewater treatment tank so that the surfaces are alternately exposed to the wastewater and to oxygen in the ambient air. Organisms in the biomass remove dissolved oxygen and organic materials from the film of wastewater. Unused dissolved oxygen in the wastewater film is mixed with the contents of the mixed liquor in the tank.
It has been common to drive the contactors by use of a motor, usually electric, connected through a reduction gearing to the horizontal shaft upon which the contactor rotates. The amount of force required to rotate the RBC is critical, for the biomass has a tendency to accumulate on the myriad surfaces of the contactor media to such an extent that a significant supplemental load is placed on the contactor structure, shaft and bearings, thus impeding the rotation of the device and exerting the major structural load on the RBC apparatus.
This process of biomass loading is accentuated as the diameter of the contactor increases. It is now common practice to install RBCs having diameters on the order of twelve feet.
Increased diameter RBCs are capable of substantially reducing manufacturing and installation costs, provided the structural loads do not become excessive. One way of reducing biomass loading is through the use of supplemental aeration which provides additional control of the thickness and the type of biomass which grows on the RBC surface.
Supplemental aeration may serve a dual purpose, for U.S. Pat. No. 3,886,074 to Prosser teaches the use of air capture devices mounted on the media to capture some of the supplemental air and cause the RBC to rotate. This apparatus eliminates the need for a direct electrical or mechanical drive system to be applied to rotate each RBC as is conventional practice. One disadvantage of the use of air capture devices when applied to conventional submergence RBCs (those having 40-50% submergence), is the need for excessive amounts of air to maintain rotation once the biomass grows in an uneven fashion, thus causing substantial structural imbalance. For the purposes of this application, "percent submergence" refers to the amount of submerged media surface area.
Further, it takes substantial energy to rotate a loaded RBC when immersed only 40-50% in the wastewater due to the significant amount of wastewater which is drawn into the air, plus the additional drag imposed by the air capture devices mounted to the media periphery as they reenter the wastewater. The air capture devices disclosed by Prosser are purposely designed to trap air in order to rotate the RBC unit. A disadvantage of this construction is that as a specific point on the RBC unit rotates through the air and begins to descend into the water, the design of the air capture device creates a drag on the rotational velocity of the RBC.
A still further consideration of RBC construction is that RBCs are often used to increase the efficiency of existing primary or secondary (activated) sludge sewage treatment plants. This is accomplished by installing conventional RBCs, typically 11-12 feet in diameter, in the existing clarifier and/or aeration tanks. Since these tanks may normally be 10-20 feet in liquid depth, conventional RBCs at normal 40% submergence require substantial modification to the existing tanks. Usually a new tank bottom is required at a higher elevation located only 5-7 feet below the top of the tank and just below the RBC media.
This method, while currently a cost effective method to upgrade plant capacity and performance, has the disadvantage that the entire tank volume cannot be used effectively and requires substantial construction costs to provide the new false bottom.
U.S. Pat. No. 3,704,783 to Antonie discloses the use of a combination of partially and totally submerged RBCs to aerate a secondary treatment tank. The Antonie system includes partially submerged RBCs which provide a supply of aerated water to the totally submerged RBCs. Subsequent practice has shown that this method of adding dissolved oxygen to the wastewater is not sufficient to permit submerged media surfaces to function aerobically. A further means to reduce flow velocity and induce mixing and turbulence within the submerged media is necessary to maintain aerobic conditions.
Thus, there is a need for an RBC apparatus which provides for the deep submergence of larger diameter RBC units into both new and upgraded secondary treatment tanks so that a substantial amount of the contained water is treated by the RBC, the attached RBC biomass receives sufficient oxygen and a minimal amount of energy is required to aerate and rotate the RBC.
It is therefore an object of the present invention to provide a large diameter, deep submergence RBC apparatus.
It is another object of the present invention to provide a deep submergence RBC apparatus which can be installed in existing tankage without extensive modifications.
It is a further object of the present invention to provide a large diameter, deep submergence RBC apparatus which does not require elaborate drive means.
It is a still further object of the present invention to provide a large diameter, deep submergence RBC apparatus which provides for adequate aeration, mixing and thickness control for an attached, constantly submerged biomass, and a means of providing control of rotational velocity.